When sensors located at a number of points are connected to each other via a network, information obtained from the respective sensors can serve for control and security of an information device and monitoring of an environment. For example, large-scale structures such as a high-rise building and a bridge are required to have a life of several decades. The repair of such structures before being damaged, rather than after being damaged seriously, can be done at lower cost with a higher degree of safety. In view of this, it is considered to install an acceleration sensor or a strain sensor in such a structure so as to perform long-term monitoring, thereby predicting damage to be caused thereto and the like. Also, it is considered to attach a sensor to person's clothing, belongings, or appliances so as to record its position, temperature, and the like, thereby managing goods, predicting person's behavior, performing environmental measurement, and the like.
When long-term observations are performed using a sensor as described above, a conventional battery or cell does not serve practically as a power source for the sensor because it provides only a short operation period and costs more for replacement.
When the sensor is operated intermittently, the operation can continue for a long term with several μW of electric power. As a method for obtaining such electric power, there is a method of collecting vibration energy in an environment where the sensor is attached as electric energy.
As a power generating mechanism that collects vibration energy as electric energy, a power generating mechanism using a piezoelectric material as described in Patent Document 1 is known. When a piezoelectric material is strained, an electric charge is generated in the polarization direction, resulting in the generation of an electric field. With the use of such a material, mechanical vibration energy can be converted into electric energy. Patent Document 1 discloses a power generating mechanism in which one end of a piezoelectric material shaped into a strip is fixed, and a mass is attached to the other end thereof, so that the piezoelectric material is bent and deformed due to external vibrations, thereby generating electricity.
In order to generate electricity efficiently with such a vibration-type power generating mechanism, it is required that the oscillation frequency of vibration energy to be collected and the resonant frequency of the power generating mechanism be approximated to each other. More specifically, it is necessary to analyze vibrations to be collected as electric energy, so that the resonant frequency of the power generating mechanism is set in accordance with the analysis.
For example, FIG. 15A shows a change with time in acceleration caused when a human is walking, and FIG. 15B shows frequency components of acceleration caused when a human is walking. As shown in FIG. 15B, vibrations generated when a human is walking include a large number of low frequency components of not more than several Hz. Thus, in order to generate electricity by body movements of a human while he/she is walking, it is necessary to use a power generating mechanism with a resonant frequency as low as the above.
Further, structures such as a building and a bridge also are vibrated due to the wind or a movable body passing in the vicinity thereof. Since a structure itself is large in size, its oscillation frequency is known to be as low as approximately several Hz. Thus, a power generating mechanism with a low resonant frequency is required also in the case of collecting electric energy from vibrations of a structure.
Meanwhile, when a power generating mechanism is large in size, the place where a sensor including the power generating mechanism is to be attached is limited. For this reason, a power generating mechanism is preferably as small in size as possible.    Patent Document 1: JP 2005-45988 A